I'm also in my 3rd pregnancy with 2 prior sections and I get this pain almost every time I get up from a laying position. It is excrutiating! However I also had it late in my first pregnancy and was told that it was round ligament pain. I haven't put any more thought into it than that. I try to remember to stretch out (like arms over my head and stretch through my toes) before I attempt to get up, and then get up slowly. This seems to help a little. I do agree with pp, if it's new you should talk to your doctor about it